Molecular Dynamics Simulations Of Nucleation And Stability Of Interfacial Nanobubble

Existence of interfacial nanobubbles that preferentially nucleate on hydrophobic solid surfaces immersed in solutions with dissolved gas has been recently verified experimentally.The tiny bubbles show a variety of properties that are different from their macroscopic counterparts.They have the potentialwith a variety of applications in surface physics,chemistry,biology and industry.For example,interfacial nanobubbles can reduce the flow resistance of nanofluids,increase the recovery of mineral flotation,remove surface contaminants,and use ultrasound irradiation for imaging tumor.In experimental studies,nanobubbles are mainly prepared by solvent exchange and electrolysis techniques.However,the factors affecting the formation of bubbles,such as gas supersaturation,solvent exchange rate,and fluid shear rate,are numerous.So that it is difficult for the experimental studies on nanobubble nucleation andon the controllable formation of nanobubbles.A large number of experimental and theoretical studies have shown that the nanobubbles are very stable,and the proposed theory of contact line pinning and supersaturation can also explain the stability of the bubble stability.However,the thermodynamicstudy of the stability of nanobubbles is not yet mature,and the reason of how nanobubbles lose stability under various influencing factors is far from being understood.Therefore,this dissertation develops theoretical and simulation studies on various problems in the nucleation and stability of surface nanobubbles.The main contents are as follows:1.The relationship between the curvature radius of surface nanobubbles and the critical nucleus radius of the bubble nucleation.In this part,using classical nucleation theory and molecular dynamics simulation weexplore the relationship.We first use a simple geometric relation to show that we can obtain information about the homogeneous nucleation process from Molecular Dynamics studies of bubble formation in solvophobic nanopores on a solid surface.The free energy of pinned nanobubbles has two extrema as a function of volume:one state corresponds to a free-energy maximum("the critical nucleus"),the other corresponds to a free-energy minimum(the metastable,pinned nanobubble).Provided that the surface tension does not depend on nanobubble curvature,the radius of the curvature of the metastable surface nanobubble is independent of the radius of the pore and is equal to the radius of the critical nucleus in homogenous bubble nucleation.Our theoretical analysis also indicates that a surface with pores of different sizes can be used to determine the curvature corrections to the surface tension.2.The nucleation mechanism of nanobubbles during solvent exchange.Our molecular simulations indicate that a solvent exchange process can be divided into two stages.At the first stage of solvent exchange,there exists an interface between interchanging solvents of different gas solubility.This interface moves toward the substrate gradually as the exchange process proceeds.Our simulations reveal directed diffusion of gas molecules against the gas concentration gradient,driven by the solubility gradient of the liquid composition across the moving solvent-solvent interface.It is this directed diffusion that causes gas retention and produces a local gas oversaturation much higher near the substrate than far from it.At the second stage of solvent exchange,the high local gas oversaturation leads to bubble nucleation either on the solid surface or in the bulk solution,which is found to depend on the substrate hydrophobicity and the degree of local gas oversaturation.3.The nucleation mechanism of nanobubbles in electrolysis.Our molecular dynamics simulations show that either the electrode radius and the electrolysis rate during the electrolysis of nanobubbles have a significant effect on the formation of nanobubbles.As the electrode radius and electrolysis rate increase,the form of bubble nucleation will change from single nucleus to multiple nuclei.For multiple nanobubbles,they eventually merge into one bubble during the bubble growth.The bubbles forming processes undergo a process frompartial surface coverage,to whole coverage and thento the partial coverage.This process corresponds to the fact that the amount of gas generated by electrolysis at the first stage is greater than the amount of gas diffused into the environment,leading to the nanobubble formation and growth on the electrode,which in turn decrease of the area left for electrolysis.At the second stage,as the uncovered electrode area is minimized,the nanobubble would begin to decrease.When the bubble shrinks to less than the radius of the electrode,the amount of gas generated will reach a dynamic equilibrium with that diffused into the environment,thereby forming a stable nanobubble under dynamic equilibrium conditions.4.The molecular mechanism ofhow nanobubbles lose stability under the influence of surfactants.Our molecular dynamics simulations indicate that there exist two surfactant-induced molecular mechanisms for nanobubbles losing stability,either through depinning of a contact line or reducing vapor-liquid surface tension.One corresponds to the case with significant adsorption of surfactants on the substrates,which causes depinning of the nanobubble contact line and thus leads to nanobubble instability.The other stresses surfactant adsorption on the vapor-liquid interface of nanobubbles,especially for insoluble surfactants,which reduces the surface tension of the interface and leads to an irreversible liquid-to-vapor phase transition.